FIG. 1 is a hydraulic circuit diagram of a working device control device for a construction machine of the prior art.
As illustrated in FIG. 1, first and second variable displacement hydraulic pumps (hereinafter referred to as first and second hydraulic pumps) 2 and 3 are connected to an engine 1 or the like.
First and second hydraulic control levers 4 and 5 output control signals corresponding to the degree of manipulation.
A first working device (not shown) is actuated by hydraulic fluid supplied through a supply passage 6 by the first hydraulic pump 2.
A second working device (not shown) is actuated by hydraulic fluid supplied through a supply passage 8 by the second hydraulic pump 3.
A first working device control valve 7 is disposed on the supply passage 6 between the first hydraulic pump 2 and the first working device. When the first working device control valve 7 is switched by pilot pressure applied by the first hydraulic control lever 4, the first working device control valve 7 controls the direction and flow rate of hydraulic fluid supplied to the first working device.
A second working device control valve 9 is disposed on the supply passage 8 between the second hydraulic pump 3 and the second working device. When the second working device control valve 9 is switched by pilot pressure applied by the second hydraulic control lever 5, the second working device control valve 9 controls the direction and flow rate of hydraulic fluid supplied to the second working device.
A confluence valve 10 is disposed on the supply passage 8, upstream of the second working device control valve 9. When the confluence valve 10 is switched by pilot pressure applied by the first hydraulic control lever 4 so that the first and second working devices perform complex operations, the confluence valve 10 allows a portion of hydraulic fluid discharged from the second hydraulic pump 3 to flow through a confluence passage 14 to join hydraulic fluid discharged from the first hydraulic pump 2.
A confluence shut-off valve 12 is disposed on a pilot line 11 through which pilot pressure is applied. When the confluence shut-off valve 12 is switched by pilot pressure applied by the second hydraulic control lever 5, the confluence shut-off valve 12 shuts off the supply of pilot pressure to the confluence valve 10.
Reference numeral 13 in the drawings, while not referred to, is a main control valve (MCV).
When the first hydraulic control lever 4 is manipulated to operate the first working device, a spool is switched to the right, as depicted in the drawing, by pilot pressure applied to the first working device control valve 7. Subsequently, hydraulic fluid supplied by the first hydraulic pump 2 is supplied to a hydraulic cylinder for the first working device through the supply passage 6, the switched first working device control valve 7, and a passage 14a. 
When the flow rate of hydraulic fluid required by the first working device is greater than the maximum flow rate of hydraulic fluid supplied by the first hydraulic pump 2, a portion of hydraulic fluid discharged from the second hydraulic pump 3 is allowed to join hydraulic fluid discharged from the first hydraulic pump 2. That is, in response to the first hydraulic control lever 4 being manipulated, pilot pressure is applied to the first working device control valve 7. At the same time, in response to the first hydraulic control lever 4 being manipulated, pilot pressure is applied to the confluence valve 10 through the pilot line 11 and then through the confluence shut-off valve 12. Here, the spool of the confluence valve 10 is switched to the left, as depicted in the drawing.
As illustrated in FIG. 5, the spool of the confluence valve 10 is not switched when pilot pressure from the first hydraulic control lever 4 is lower than the joining point and is switched when the pilot pressure becomes equal to or greater than the joining point. Thus, hydraulic fluid discharged from the second hydraulic pump 3 flows through the supply passage 8, an inner passage 10a of the switched confluence valve 10, and the confluence passage 14, and then in the passage 14a, joins hydraulic fluid that has been supplied by the first hydraulic pump 2 through the first working device control valve 7.
When the spool of the second working device control valve 9 is switched by pilot pressure applied by the second hydraulic control lever 5 to operate the second working device, the second working device control valve 9 is supplied with an insufficient amount of hydraulic fluid from the second hydraulic pump 3, since a portion of hydraulic fluid discharged from the second hydraulic pump 3 has been supplied for the first working device in response to the switching of the confluence valve 10.
When the second hydraulic control lever 5 is manipulated to prevent this problem, pilot pressure is applied to the second working device control valve 9 and the confluence shut-off valve 12. Here, the spool of the confluence shut-off valve 12 is switched downwardly, as depicted in the drawing.
Thus, it is possible to ensure that hydraulic fluid is supplied to the second working device by the second hydraulic pump 3 by shutting off pilot pressure applied to the confluence valve 10 by the first hydraulic control lever 4 to be proportional to pilot pressure applied by the second hydraulic control lever 5.
Here, according to the characteristics of the spool of the confluence shut-off valve 12, in a transition period before the spool is switched to a full open state, pilot pressure is not formed in the pilot line 11 to a level desired by an operator, for various reasons, such as internal leakage or communication with a hydraulic fluid tank.
In addition, a pilot line, through which pilot pressure is applied by the first hydraulic control lever 4 to the first working device control valve 7, communicates with the pilot line 11. This causes pressure loss due to internal leakage or the like. Consequently, the operator cannot control the first working device as accurately as he or she may desire.
In addition, the confluence shut-off valve 12 is configured such that the spool thereof can be mechanically controlled. Once the confluence shut-off valve 12 is assembled by setting the open area of the spool, it is difficult to adjust the open area. In addition, in construction machines, it is difficult to realize an approach of variably controlling the spool of the confluence valve 10 using pilot pressure by manipulating the first and second hydraulic control levers 4 and 5, which is problematic.
In addition, in the confluence valve 10, the control period of the right portion of the spool (to be used in joining for the first working device when the spool is switched) is different from the control period of the left portion of the spool (to be used in control over the other working device). Thus, valve springs on the right and left of the spool are required to have different specifications.
FIG. 2 is a hydraulic circuit diagram of another working device control device for a construction machine of the prior art.
As illustrated in FIG. 2, first and second variable displacement hydraulic pumps (hereinafter referred to as first and second hydraulic pumps) 2 and 3 are connected to an engine 1 or the like.
First and second hydraulic control levers 4 and 5 output control signals corresponding to the degree of manipulation.
A first working device (not shown) is actuated by hydraulic fluid supplied through a supply passage 6 by the first hydraulic pump 2.
A second working device (not shown) is actuated by hydraulic fluid supplied through a supply passage 8 by the second hydraulic pump 3.
A first working device control valve 7 is disposed on the supply passage 6 between the first hydraulic pump 2 and the first working device. When the first working device control valve 7 is switched by pilot pressure applied by the first hydraulic control lever 4, the first working device control valve 7 controls the direction and flow rate of hydraulic fluid supplied to the first working device.
A second working device control valve 9 is disposed on the supply passage 8 between the second hydraulic pump 3 and the second working device. When the second working device control valve 9 is switched by pilot pressure applied by the second hydraulic control lever 5, the second working device control valve 9 controls the direction and flow rate of hydraulic fluid supplied to the second working device.
A confluence valve 10 is disposed on the supply passage 8, upstream of the second working device control valve 9. When the confluence valve 10 is switched by pilot pressure applied by the first hydraulic control lever 4 so that the first and second working devices perform complex operations, the confluence valve 10 allows a portion of hydraulic fluid discharged from the second hydraulic pump 3 to flow through a confluence passage 14 to join hydraulic fluid discharged from the first hydraulic pump 2.
A first pressure sensor 15 detects the level of pilot pressure applied to the second working device control valve 9 from the second hydraulic control lever 5.
A first proportional control valve 16 is disposed on a pilot line 11 through which pilot pressure is applied to the confluence valve 10 by the first hydraulic control lever 4. The first proportional control valve 16 converts pilot pressure, formed in response to the first hydraulic control lever 4 being manipulated, to have a secondary pressure, proportional to an electrical signal applied to the first proportional control valve 16 and applies the secondary pressure to the confluence valve 10.
A controller 17 has a detection signal input thereto by the first pressure sensor 15. The controller 17 applies an electrical signal to the first proportional control valve 16 so that a control signal, calculated to be inversely proportional to the input detection signal, can be applied to the confluence valve 10.
Thus, when the first hydraulic control lever 4 is manipulated to operate the first working device, the spool of the first working device control valve 7 is switched to the right, as depicted in the drawing, by pilot pressure applied thereto. At the same time, pilot pressure formed in response to the first hydraulic control lever 4 being manipulated is converted to a secondary pilot pressure in response to an electrical signal applied to the first proportional control valve 16 by the controller 17. The level of secondary pressure is applied to the confluence valve 10 through the pilot line 11.
Here, when the spool of the second working device control valve 9 is switched in response to the second hydraulic control lever 5 being manipulated to operate the second working device, pilot pressure applied to the second working device control valve 9 is detected by the first pressure sensor 15 and a detection signal is input to the controller 17.
At this time, the controller 17 applies an electrical signal to the first proportional control valve 16 such that the electrical signal is inversely proportional to the level of pilot pressure applied to the second working device control valve 9. Thus, a secondary pilot pressure formed by the first proportional control valve 16 reduces pilot pressure which is otherwise applied to the confluence valve 10 by the first hydraulic control lever 4.
Consequently, the open area of the spool of the confluence valve 10 can be variably controlled depending on the level of pilot pressure applied by the second hydraulic control lever 5 to the second working device control valve 9.
In addition, a pilot line, through which pilot pressure is applied by the first hydraulic control lever 4 to the first working device control valve 7, communicates with the pilot line 11, through which pilot pressure is applied to the confluence valve 10 by the first hydraulic control lever 4. This causes pressure loss due to internal leakage or the like. Consequently, the operator cannot control the first working device as accurately as he or she desires.
In addition, in the case of switching the spool of the first working device control valve 7 by manipulating the first hydraulic control lever 4, a portion of pilot pressure for switching the first working device control valve 7 influences the spool of the confluence valve 10 through the pilot line 11. In such a case, it is impossible to precisely manipulate the first and second working devices, which is problematic.
Furthermore, since pilot pressure formed by manipulating the first hydraulic control lever 4 is used as pilot pressure supplied to the first proportional control valve 16, a secondary pilot pressure formed by the first proportional control valve 16 cannot be arbitrarily changed. Thus, right and left valve springs of the spool of the confluence valve 10 are required to have different specifications.